Fluorine

No element pulls electrons more aggressively toward itself than fluorine. With an electronegativity of 3.98 on the Pauling scale — the highest of any element — and an ionization energy of 17.4 electron volts, fluorine is the most chemically reactive nonmetal on the periodic table. At room temperature it is a pale yellow diatomic gas that attacks glass, ignites asbestos, and reacts explosively with water. Its bite is so powerful that even so-called noble gases xenon and krypton yield compounds under its influence. The tight, high-energy F-F bond in F2 breaks easily, releasing two fluorine radicals that will bond to almost anything in their path. Yet paradoxically, once fluorine bonds to carbon, it creates one of the strongest and most stable linkages in all of chemistry — a combination exploited in drugs, refrigerants, and polymers that defines modern materials science.

Fluorine's largest industrial outlet is the production of hydrofluoric acid, HF, which etches silicon wafers in semiconductor fabrication and processes uranium ore into uranium hexafluoride for nuclear fuel enrichment. Chlorofluorocarbons, the now largely phased-out refrigerants implicated in ozone depletion, and their hydrofluorocarbon replacements, rely on carbon-fluorine bonds for stability. Polytetrafluoroethylene, PTFE — sold commercially as Teflon — coats non-stick cookware and industrial pipe linings with a near-frictionless, chemically inert surface that nothing wants to stick to. Fluoride added to municipal drinking water and toothpaste at around one part per million hardens tooth enamel by converting hydroxyapatite to the more acid-resistant fluorapatite, significantly reducing cavity rates. Fluorine atoms incorporated into pharmaceutical molecules — from antidepressants like fluoxetine to antibiotic fluoroquinolones — slow metabolic degradation, improve membrane permeability, and fine-tune binding to biological targets. Roughly one-quarter of all drugs approved in recent decades contain at least one C-F bond.

Hydrofluoric acid's corrosive power was documented as early as 1670, when Schwanhard discovered it could etch glass. Chemists recognized fluorine as a distinct element by the early nineteenth century and correctly predicted its position in the halogen family, but its isolation proved disastrously difficult. The element is so reactive that every attempt to free it from its compounds either destroyed the apparatus or incapacitated the experimenter. Humphry Davy, Edmond Fremy, and the Knox brothers all suffered serious injuries or prolonged illness from fluorine exposure during their attempts. The element claimed several lives directly. It was Henri Moissan who finally succeeded in 1886, isolating fluorine by electrolyzing a cooled solution of potassium bifluoride in anhydrous hydrofluoric acid using a platinum apparatus. He was awarded the Nobel Prize in Chemistry in 1906 specifically for this achievement. The recognition noted that fluorine had resisted all previous attempts and that Moissan's success represented one of the most challenging isolations in the history of chemistry.

Fluorine is the thirteenth most abundant element in Earth's crust at roughly 600 to 700 parts per million, but free elemental fluorine F2 does not occur in nature — its reactivity ensures it is always found bound to other elements. The principal commercial mineral is fluorite, calcium fluoride (CaF2), a cubic crystal that occurs in colorful veins associated with granite and hydrothermal ore deposits. Its name comes from the Latin fluere, to flow, because it was used as a flux in smelting. Fluorapatite, a calcium fluoride phosphate, is the main fluorine-bearing mineral in phosphate rocks and is also the form that makes up tooth enamel and bone mineral. Cryolite, sodium aluminum fluoride (Na3AlF6), once mined in Greenland and now synthesized, was historically essential as a solvent for aluminum smelting. Seawater contains about 1.3 parts per million of fluoride ion. Small amounts of organofluorine compounds are produced by certain plants and microbes, but these biological sources are negligible compared to mineral reserves.

Hydrofluoric acid, HF, is the most commercially important fluorine compound, serving as both an industrial etchant and the feedstock for nearly all other fluorine chemistry. Calcium fluoride, CaF2, or fluorite, is the primary mineral ore and is also used as an optical material transparent to ultraviolet and infrared radiation. Sulfur hexafluoride, SF6, is an extremely dense, chemically inert gas used as an electrical insulator in high-voltage circuit breakers, though its potent greenhouse warming effect has prompted efforts to reduce its use. Polytetrafluoroethylene, (CF2-CF2)n, better known as PTFE or Teflon, represents the endpoint of fluorine's ability to stabilize carbon chains; the resulting polymer is inert to virtually every chemical at ordinary temperatures. Fluorapatite, Ca5(PO4)3F, forms when fluoride replaces hydroxide in hydroxyapatite, the mineral phase of bones and teeth, making enamel significantly more resistant to acid dissolution. Uranium hexafluoride, UF6, is the gaseous form used in uranium enrichment by gaseous diffusion and centrifuge processes.

• Fluorine is the only element that can displace oxygen from water — it reacts with H2O to produce hydrogen fluoride and oxygen gas, a reaction so vigorous it occurs spontaneously at room temperature.
• Henri Moissan's hard-won isolation of fluorine in 1886 came at a personal cost: repeated exposures throughout his career are believed to have shortened his life significantly, and he died just months after receiving his Nobel Prize at age 54.
• The carbon-fluorine bond is the strongest bond that carbon forms with any other element, with a bond dissociation energy of roughly 544 kilojoules per mole — which is why PTFE-coated pans essentially never react with food or cleaning agents.
• Fluoride at about one part per million in drinking water reduces childhood tooth decay by roughly 25 percent, one of the most cost-effective public health interventions ever documented, yet the same fluoride at much higher concentrations causes dental and skeletal fluorosis.
• About 20 percent of all pharmaceutical drugs on the market today contain at least one fluorine atom, including Prozac, Lipitor, Cipro, and Diflucan — a proportion that has roughly tripled since the 1970s as medicinal chemists learned to exploit fluorine's metabolic-stabilizing effects.